Fixation of prosthetic flexible tension members, such as tendons or ligaments, to relatively rigid structures is a serious problem. A notable example is the use of artificial ligaments, such as the Leeds-Keio anterior cruciate ligament replacement in the knee. In that example, published experience with the usual means of bone fixation—drilling a hole in the tibia, inserting the ligament, and securing with a suture or pin—has included several instances of fragmentation of the polyester fibers of the prosthesis within a few months to a few years. A compression plate fixation has been used whereby tension members are cut and the end grasped between two plates, generally textured and held together by compression screws to grasp the tension member. While this allows greater control of local stress concentration than does a simple bone-hole, in theory it delivers extremely high shear stresses to the tension member locally, which may cause fatigue failure and breakage over the immense number of stress cycles expected to be required.
A knob-loop fixation device has been previously disclosed to address the stress-concentration issue, but requires a substantial thickness that may be disadvantageous. Such thickness could be problematic in some cardiac surgical, plastic and reconstructive surgical, or orthopaedic surgical devices, for example, in regions where skin is quite close to a coupled bone (e.g., the frontal bone in the case of a cosmetic surgical ‘brow lift’ prosthesis and the olecrenon in an orthopaedic surgical elbow prosthesis). Further, for either of these applications, for other plastic and orthopaedic surgical applications, or for some potential configurations of mechanical energy converters for cardiac power applications, the surface to which the coupler is attached may vary in its contour. Therefore a much thinner adapting terminus, which maintains sufficient flexibility to allow a finite number of size/shape models to conform to anatomy of reasonable individual variation, would be of benefit. Further, a structure with a soft flexible interface to fibers (reducing stress concentration) and yet a harder external surface (to interface with other tissues, adhering or not as desired) would also be advantageous.
Natural tendon ends, which are living tissue, have been connected to ‘towel bar’ fixtures on artificial bones, over which they are looped and sewn. Because of the shape of tendons—generally flattened in the plane of attachment, the axis of curvature is generally perpendicular to the surface to which they are to be attached. To avoid intolerable protrusion dimensions into surrounding tissue structures, the radius of curvature is very small. Since the compressive stress on a tension member surface, when that tension member is looped about any rod or pulley, is directly proportional to the tension applied and inversely proportional to both the radius of curvature and the projection of contact surface perpendicular to the transmitted tension, compressive forces intolerable by the tension member may be generated. An artificial force transmitting tension member, however, such as an artificial tendon, can be formed in any cross-sectional configuration. This allows the central stabilizing point to be relatively thin, flat, and oriented in the plane of the surface to which the tension member is to be attached.
In contrast to the ‘towel bar’ concept, the radius of curvature of the present invention may be made substantially larger with only minimal protrusion into surrounding tissue structures. In contrast to the ‘knob loop’ or ‘tangential pulley’ concept, the present invention does not require fibers to be organized into a circular cross-section, with imposition of a minimum thickness for a given number of fibers. The number of fibers still dictates the cross-sectional area of the bundle that passes through the matrix, but it can be very wide and quite thin, or any other combination of dimensions dictated by the device (e.g., a mechanical energy converter) or anatomic structure (e.g., a bone) to be joined.